Testing for Stiff-Person Syndrome (SPS) antibodies typically involves a two-step process: initial screening of serum for glutamic acid decarboxylase 65-kDa isoform (GAD65) antibodies, followed by cerebrospinal fluid (CSF) analysis if serum GAD65 levels are low, to confirm the diagnosis.
Here's a more detailed breakdown:
Step-by-Step Antibody Testing for SPS
-
Initial Serum Screening for GAD65 Antibodies:
- The first step usually involves testing a blood sample (serum) for the presence and level (titer) of GAD65 antibodies. GAD65 is an enzyme involved in the production of GABA, a neurotransmitter that inhibits nerve signals. SPS is often associated with high levels of GAD65 antibodies. However, low levels warrant further investigation.
-
Cerebrospinal Fluid (CSF) Analysis:
- If serum GAD65 antibody titers are low or inconclusive, a lumbar puncture (spinal tap) may be performed to collect CSF. The CSF is then tested for GAD65 antibodies and potentially other antibodies associated with SPS, such as amphiphysin, gephyrin, or dipeptidyl-peptidase-like protein-6 (DPPX) antibodies, depending on the suspected phenotype.
Phenotype-Targeted Antibody Panels
Laboratories often utilize phenotype-targeted panels. These panels are designed to test for a range of antibodies associated with different variations or presentations within the stiff-person spectrum of disorders. This allows for a more comprehensive assessment.
Interpretation of Results
It's crucial to interpret antibody test results in the context of the patient's clinical presentation, neurological examination, and other diagnostic findings. Elevated levels of GAD65 antibodies, particularly in the CSF, strongly support a diagnosis of SPS. However, the absence of antibodies doesn't necessarily rule out the condition. Some individuals with SPS may be seronegative, meaning they do not have detectable antibodies in their serum or CSF.
